In a nuclear reactor power plant, one design of a fuel assembly is comprised of a plurality of fuel elements or rods oriented in a square-shaped array. For a typical pressurized water reactor, there are on the order of about 200 of these elongated fuel rods in each fuel assembly (see FIGS. 11 and 12). At either end of the fuel assembly is a top and bottom nozzle which direct the flow of coolant, typically water, through each fuel assembly. Interspersed among the fuel rods are hollow tubes, or thimbles, into which control rods are inserted from time to time to control the rate of nuclear reactions taking place. Positioned along the length of the fuel assembly are grids which help align and support fuel and control rods. Altogether, this fully loaded assembly can weigh on the order of 545 kg. (1200 lbs.).
To provide for a singular unit, the ends of the control rod guide tubes or thimbles are secured to the top and bottom nozzles. To accomplish this, typically, a stainless steel tube projecting from a nozzle is secured around each of the control rod guide thimbles. The guide thimbles are made of a special material which does not readily absorb neutrons developed by Westinghouse, Zircaloy. To secure the top nozzle to these guide thimbles, conventional methods such as welding cannot be used. Thus the thimble tubes are secured to the stainless steel tubes of the top nozzle by radially crimping the tubes together, usually at three axial locations, to secure the top nozzle to the fuel assembly.
Due to extended storage periods, spent fuel assemblies that have been stored in liquid, usually water, may have experienced corrosion, thereby rendering their removal by originally contemplated means impractical. Initially, the spent fuel assemblies were to have been transported in the same manner as fresh or new fuel assemblies are: via a tool which attaches to the nozzle at the top of the assembly. This method would allow the bulk of the assembly's mass to hang from the guide tubes, which are suspected of corrosion. The crimped segments below the nozzle are at risk of being torn by the weight of the assembly itself, if the assembly is lifted by the nozzle. Various tools and methods have been developed to transport a fuel assembly skeleton, or one which has had the nozzles and fuel rods removed therefrom, but are not capable of handling a fully loaded fuel assembly. Since these spent fuel assemblies are stored in a spent fuel pool which can be about 10 m (32 ft.) deep, an operator of such a tool should be able to be confident that a fuel assembly on the remote end has been effectively gripped for safe handling. Because of this risk, a tool capable of lifting the full weight of the assembly, without overly stressing the crimped segment, is desirable.
It is thus an object of the present invention to provide an apparatus which is capable of lifting and transporting fully loaded fuel assemblies.
It is another object of the present invention to provide such a lifting device which will not overly stress components of the fuel assembly.
It is a further object of the present invention to provide an apparatus which will give a positive indication to an operator of a correct positioning of the device.
It is a still further object of the present invention to provide an apparatus which will indicate to the operator when a fuel assembly has been properly gripped, and to continue to monitor the integrity of the grip while transporting a fuel assembly.